The chemistry of the molybdenum cofactor present in sulfite oxidase and xanthine oxidase is yet unknown. Since molybdate will not activate the apoproteins in vitro but does accomplish it in vivo, an enzymatic modification needs to be defined, and the pertinent enzyme or enzymes has to be sought for. The presence of a hitherto unidentified molybdenum component in rat liver was discovered during our studies on tungsten treatment. The identity of this component has to be established, in order that all aspects of the nutritional essentiality of molybdenum may be understood. While sulfite oxidase activity is not elevated in adult animals on exposure to SO2 and bisulfite, it is conceivable that one or both of these chemicals will induce the enzyme in growing animals whose tissues contain low levels of sulfite oxidase. The possibility that high sulfur diets will produce symptoms of toxicity in tungsten-treated animals lacking sulfite oxidase will be explored.